Project Summary Myeloid cells, including monocytes and macrophages, are classically activated to clear pathogens and promote immunity, but these same cells can also be reprogrammed within the tumor microenvironment to become tumor-associated macrophages (TAMs), where they suppress anti-tumor immunity and promote tumor growth and metastasis. Studies investigating the biological mechanisms behind anti-tumor ?M1-like? classical maturation versus tumor-promoting ?M2-like? alternative activation possess significant therapeutic potential. Paired Ig-like receptor B (PIR-B), a receptor mainly expressed on myeloid lineages in mice, suppresses immune activation. The leukocyte immunoglobulin-like receptor subfamily B (LILRB) represents the human ortholog of mouse PIR-B. We found that PIR-B maintains the M2-like phenotype typical of tumor infiltrating myeloid cells. Mice deficient in PIR-B have reduced tumor burdens and an infiltrating MDSC profile that resembles the M1-like classical activation phenotype. Therefore, modulation of LILRB signaling in myeloid cells may provide a means for controlling tumor invasion/progression. Our group generated a panel of antibodies against LILRB family members and screened them for functional activity in human monocyte-derived macrophage cell-based assays. Consistent with our findings in mice, we observed that LILRB antagonistic antibodies promote classical M1-like activation. Concordantly, macrophages down-regulate M2-associated lpha while suppressing IL-10 secretion, a profile consistent with M1 classical maturation. We hypothesize that LILRB is an important homeostatic regulator that suppresses human monocyte classical activation, thereby playing a key role in tumor progression and metastases. Three specific aims will be pursued: 1) Modulate the function of myeloid cells through PIRB/LILRB to promote anti-tumor responses. 2) LILRB controls tumor invasion. 3) Prevent tumor invasion/progression by fostering M1 macrophage differentiation as an immune checkpoint therapy. Studies of the cellular and molecular mechanisms of action utilized by LILRB are critical for clinical translation. Successful completion of these studies will lead to a better understanding of how our findings with mouse PIRB can be translated to human LILRB. Furthermore, LILRB blockade can alter the tumor microenvironment, enhance anti-tumor immunity, and control tumor cell invasion. These findings may lead to the discovery of novel means by which TAMs/MDSC can be targeted to combat the immune suppression associated with advanced malignancies. Ablation of immune suppression and preventing tumor invasion should significantly augment the efficacy of immune-based therapies for the treatment of advanced metastatic cancer.